Multiplexed Sensors Monitor Bridge Structure, Materials

The Taylor Bridge, which spans the Assiniboine River west of Winnipeg, Man-itoba, Canada, is a "smart structure." The two-lane 165.1-m bridge is outfitted with 66 fiber optic sensors and leading-edge calibration and measurement systems designed to monitor its structure and the materials from which it is constructed.

An engineer embeds one of 66 fiber optic sensors that will monitor the structure and integrity of advanced materials used to construct the Taylor Bridge.The bridge incorporates advanced building materials -- prestressed girders of composites including carbon-fiber-reinforced and glass-fiber-reinforced polymer. The materials are estimated to be one-fifth the weight of conventional reinforcing steel and up to six times stronger.

Embedded in the girders to measure strain, loading and temperature are 64 single and two multiplexed fiber optic sensors called Bragg gratings. They can communicate information to highway engineers using a fiber optic grating strain indicator, which manufacturer ElectroPhotonics Corp. has dubbed FOGSI. The device is inherently immune to electromagnetic interference. Using a telephone line, engineers can query the sensors and compile their data. The two multiplexed sensors, which are fiber optic cables with three sensors written into the fiber optic core, are experimental and will be read by a serial multiplexing device still under development.

Active remote monitoring

The instrumentation, which was developed by ElectroPhotonics in collaboration with the Fiber Optic Sensing Laboratory of the University of Toronto Institute for Aerospace Studies in 1993, will allow highway engineers to monitor the behavior of the new girders remotely and in real time and also to compare them with conventional girders in the bridge, which also have fiber optic sensors. In addition, it will enable dynamic sensing of singular overweight events. When the Manitoba Highways and Transportation Department allows large, overweight loads to travel across the bridge, the engineers will be able to see the response of the structure in real time. "This is a first for Manitoba," said project engineer Ruth Eden, "and it's a turning point for civil engineering in terms of new materials and instant information."

The Institute for Aerospace Studies is one of 12 universities conducting research under the auspices of Intelligent Sensing for Innovative Structures Canada, also known as ISIS Canada, one of 14 federally funded centers of excellence. ElectroPhotonics is an industrial member. Research is under way to develop demodulation devices that will read and compile data from a number of sensors simultaneously.